In U.S. Pat. No. 3,444,955, there is disclosed an interference tube for a narrow directional microphone wherein a plurality of openings are provided along an axial direction. The diameter of each successive opening on the sound collecting side is diminished the further away the opening is from the microphone unit. Thus, the acoustic resistance of the microphone gradually increases along the length of the tube. A cover member of an acoustic resistive material covers each opening, and its position over the opening can be changed so that a narrow directional flat frequency and good sound quality is achieved. However, as the orientation can be adjusted only by the openings and the covering acoustic resistive materials, the lowest interference frequency is limited by the length of the tube. To obtain a narrow directional orientation, the tube length should be elongated, which can be inconvenient.